1. Field of the Invention
The present invention is a laser based solution for non-invasive high-speed testing of multi-chip modules (MCMs) and more particularly to using a chromophore-doped polyimide as an interlayer dielectric in a multi-level thin film metallized circuit structure. The dielectric is converted into an electro-optic material by poling the device in a strong electric field. The change in the electro-optic coefficients of the chromophore-doped polyimide in the presence of electrical signals in the circuit can be detected using a laser beam. The electro-optic interaction between the poled dielectric and the laser beam allows the strength of the internal fields within the MCM to be determined as a function of position.
2. Description of the Related Art
The ability to perform in situ testing and characterization of both integrated circuits and their interconnecting substrates in complex multi-chip packaging structures is becoming increasingly important as the packages become smaller.
Electro-optic probing relics on the change in the index of refraction of a material in the presence of an electric field (linear electro-optic effect). The linear electro-optic effect was first studied in crystalline solids belonging to crystal classes that lack a center of inversion symmetry. Gallium Arsenide (GaAs) and Indium Phosphide (InP) are examples of common semiconductor materials that exhibit this effect. Certain organic polymers when doped with non-linear moites and poled are also non-centrosymmetric and therefore exhibit the electro-optic effect. Doped organic polymers are poled by insertion into a strong electric field as the polymer is heated to near its glass transition temperature.
An electro-optic probing instrument makes point-to-point electric field measurements internal to microwave circuits instead of limiting the information to that gathered at the input or output ports of a circuit. The technique has been widely demonstrated with semiconductor substrates such as GaAs and InP. However, a substrate of silicon (Si) cannot be tested because it has a center of inversion symmetry. Although this technique is popular and polyimide is an increasingly popular organic polymer used in advanced packaging applications for high-speed circuits such as MCMs, no one has extended the application of electro-optic probing to circuit structures on polyimide and compared the results to more conventional substrates used for electro-optic probing such as GaAs and InP to demonstrate the potential to non-invasively probe circuit structures that are buried in central layers of an MCM as is proposed in the present invention.